The generation of waste, particularly solid waste has become an increasingly worrisome environmental issue. Many landfills are becoming filled to the point where additional waste cannot be deposited therein. In addition, much of today's solid waste is not readily biodegradable, implying that the waste will not decompose in a timely manner. As an alternative, incinerators have been employed to burn solid waste, so as to minimize its physical footprint. However, these incinerators burn the waste and generate air pollutants which require very extensive gas cleanup, create ash which can be hazardous and produce energy only in the form of heat which is converted into electricity.
Plasma gasifiers offer an alternative to these current approaches. Plasma gasifiers use intense electrically based heating to enhance a gasification and melting process which produces a synthesis gas (syngas) consisting of hydrogen and carbon monoxide. Inorganic material is converted into a nonleachable glass. After cleaning, the synthesis gas can be converted into a variety of liquid fuels or combusted to produce electricity. Cleaning of the synthesis gas and recovering heat from the syngas can be a key part of the process.
FIG. 1 shows a representative plasma gasifier system. The plasma gasifier system 100 includes a reactor vessel 110, which is typically refractory lined. Within the vessel 110 are two or more electrodes 120a, 120b that are in electrical communication with one or more power supplies 130. In some embodiments, one electrode is suspended from the top of the reactor vessel 110, while the other electrode 120b is located at the bottom of the vessel. The power supplies 130 create a significant electrical potential difference between the two or more electrodes, so as to create an arc between the electrodes 120a, 120b. As waste is fed into the vessel 110 via a waste handler 140, it is exposed to extreme temperatures, which serve to separate the waste into its component parts.
The bottom, or lower portion of the vessel 110 contains molten metal 145. An area above the molten material forms an inorganic slag layer 147. Gasses, such as carbon monoxide and hydrogen gas, are separated and exit the vessel though portal 150. The gas, commonly known as syngas, exits the vessel 110 at an excessive temperature. Since the gas has not been processed, it is also referred to as dirty syngas. The syngas is cooled in a scrubber unit 180 to allow other particulates in the gas, such as carbon or sulfur to precipitate out of the gas. Halogens and acidic materials are also removed from the syngas. The resulting gas is now referred to as clean syngas. The clean syngas can then be used to fuel a boiler or other device.
Despite the advantages of plasma gasifiers, one issue associated with the use of plasma gasifiers is the amount of energy used to raise the temperature of feedstocks, the syngas and the slag. This heat is then lost when the syngas is cooled as it is being cleaned. Recovery of this would increase the economic benefits of plasma gasifiers.
Therefore, in some embodiments, a regenerator or heat exchanger 160 may be used to capture the heat from the dirty syngas as it exits the vessel 110 and transfer it to another medium 170, such as to water to create steam. Heat recovery can also be used for a range of applications in the gasification train, including reducing the heating requirements for final stage removal of tars and other undesirable compounds and for use in powering a turbine. Such a turbine can be used for a variety of applications, including electricity production, and powering pumps, blowers, or compressors for separation of oxygen from air. Although stationary regenerators with extensive valving may be used, it may be advantageous to utilize a moving structure, such as a ceramic structure, due to the high temperatures involved in the process.
However, the use of heat exchangers (also known as regenerators) in waste and biomass gasification systems has been inhibited by the harsh environment in which they must operate. First, the syngas, at the point it passes through the heat exchanger, is not clean. In other words, it still contains particulates and condensables, such as tars and other impurities that can be captured and clog the heat exchanger. Once tars and other particulates collect on the heat exchanger, the flow of gas through the exchanger is compromised, thereby impacting the utility of the device. The honeycomb structures which have been considered for recuperation in other applications do not have sufficiently small microhole structures to capture the particulate matter. In addition, the syngas at this point is at extremely high temperatures, making the selection of a suitable material for a heat exchanger difficult.
In addition, many heat exchangers/regenerators operate by reversing the “hot” and “cold” streams to effectively transfer the heat collected by the exchanger. This often means that the heat exchanger media has to move or rotate to affect this reversal of streams. Movement and sealing of moving parts at high temperatures is often problematic.
Therefore, there is a need for an effective apparatus and method to utilize the heat generated within a plasma gasifier. The apparatus must not only exchange heat, but also tolerate and remove particulate buildup on its surface, while operating at extreme temperatures.